Dynamoelectric machine



May 7, 1946. V

R. H. SULLIVAN DYNAMO-ELECTRIC MACHINE Fil ed May 15, 1944 l I U 1INVENTOR Y wmvfia {1, ATTORNEV/ Patented May 7, 1946 I Raymond lllfsullivan,

, to General Motors Corporation,

a corporation of. Delaware Application May 15,1944, Serial No. 535,573

" 1 claim (01.171-228) Rochester, N. Y., assignm- This invention relatesto D. electric motorsr'g';

\ and particularly to D. C. motors which operate at high speed. A majorsource of trouble isjdlestructive sparking at the brushes due to'thebrushes disengaging from the commutator, at

high speeds, such as 10,000 to 20,000'R. P.

Within that speed range, a motor with 9.32 bar commutator will produce abrush frequency over 5,000 to over 10,000 vibrations per second. When abrush is continually vibratingon the commutator, it interrupts theinductive circuit of the" .1

motor and causes a. high inductive'voltage to be generated. The higherthis voltage, the more destructive will be the sparking at the brushes.It is practically impossible to design a brush and brush springcombination having a natural vibra: I

, tion period high enough to prevent vibration, If the brush springtension is increased beyonda certain range, the mechanical wear and thepower consumed due to friction would be'prohibitive.

The object of this invention is to reduce spark-? ing at the brushes;and this object is accomplished by means of electromagnetic dampingcircuits located on the armature core. These circuits compriseshort-circuited turns of resistance wire interlinked magnetically withthe. armature coils connected with the commutator. Each short circuitedcoil is wound into the. same core slots as the armature coil or coilswith which it is magnetically interlinked. Therefore each main armaturecoil is completely magnetically inter-;=

linked with a short circuited coil at all times. When the flow of loadcurrent is changed .or' interrupted by brush vibration destructivesparking is eliminated. This is done at the expense of efficiency; butfor certain types of service,such as,

high speed operation, efliciency is of less importance than the life ofbrushes and the-commutator. The number of short-circuitedturns equalsthe number of core slots; and eachicore slot contains active sides oftwo different short-,

circuitedturns. I

Further objects and advantages of the present invention will be apparentfrom the following description, reference being had to the accompanyingdrawing wherein a, preferred embodiment of the present invention isclearly shown.

In the drawing:

Fig. 1 is a wiring diagram of the equivalent of a conventional D. C.motor;

Fig. 2 is a wiring diagram of the equivalent of DYNAMQELECTRIC Machine"g -77: I I

Detroit, Mich...,-. I

a D. C. motor provided with short circuited coils interlinkedwiththearmature coils.

Fig. 3 is a wiring diagram of the armature having main armatmfe QilS-ZM;and short circuited coils S. I I I p The performance of a conventionalD. C. motor will first be described with reference to Fig. 1.

L1 =inductance of the armature coils M. connected with the commutatorbars.C in the manner indicated by Fig.3.

R1 :resistahce of the armature coils M.

B :indicates separable contacts representing. the circuit breaker effectdue to vibrating brushes. 1

R1 --:resistance across B.

Ea -lin voltage, the voltage impressed by a currentsource. i

When' the [brushes are on the commutator, full motor current is flowing.in the windings M. When the brushes leave the commutator, due tovibration, the current t'ends to decrease at a rate depending on thecircuit constants at the time of interruption. The value L1' and'Ri areconstants, but R2 varies with the 'amountof sparking at the brushes.When-there is'n'o sparking, R: is high.

-When-a spark forms, R2 is low. v

i0 i :current at any instant afterthe'circuitis in- :curfent attheinstant of interruption at B.

terrupted at B. I Y

=the total circuit resistance after the instant of'interruption.

t =time in seconds" after interruption.

:% =rate of change in motor current when the circuit is interrupted atl3.

:motor inductive voltage due to interruption at B. 1

i if it wrpossible' to 'o'pe'ri'the circuit at B without producing anarc, the value would be infinitely great and would cause an infinitelyhigh voltage at B in accordance with Equation 1. This is an impossiblecondition to obtain because the high inductive voltage causes the airbetween the vibrating brush and the commutator to break down and an arcis formed between the separated brush and commutator.

The current 21 in the motor circuit at any instant after the time ofinterruption is At the instant of interruption t in Equation 3 is equalto zero. This makes C in Equation 3 equal to r'='l.

Therefore, when i=0,

61=Roio+EL (4) In the case of the conventional motor represented by Fig.1,

By substituting Equation 5 in Equation 4,

1=(Ri+R2) io-l-E'L (6) Therefore, when the brush separates from thecommutator R2 is very high until an arc is formed. This is the cause ofthe high inductive voltage shown in Equation 6.

Sparking at the brushes can be reduced by inductively coupling the mainarmature coils M with short circuited coils S as represented'by Fig. 3.The equivalent of this circuit is'ShOWh in 'Fig. 2, in which L2 is thinductance of the coils S and R3 is the resistance. The circuit of Fig.2 may be simplified by putting R3 in terms of the motor circuit, Omit L2and R: from Fig. '2 and addRi indicated in dot-dash lines.

Ri=R= Am An being the turn ratio or number of turns of M coils number ofturns 'of. S coils When the circuit is interrupted at B,'the-effeotproduced by the S coils is the same-as if there were a circuit dampingresistance R4 shunted across the motor windings. The circuit of themotor windings M is dampedby a resistance R4 shunted across the motorinductive circuit instead of the shunting resistance formed byanelectric arc at the brushes.

Equation 6 thereforebecomes As R1 in a series motor is relatively small,it may be neglected.

In a D. C. motor made according to Fig. 3. if there are 16 armature cOreslots, two-coils-M per slot, and six turns per coil, the total number ofturns of the M coils equal 192. The total number of S coils If R4=l0.9and io=4,

61:01 X 10.9 X 12) +EL=523+EL Er. will add directly in the case of D. C.circuits, but will add and subtract alternately in case of A. C.circuits.

From the foregoing it is apparent that, instead of creating adestructive'arc when a brush separates from the commutator, theelectrical energy is absorbed by inducing a voltage in the shortcircuited turns. Therefore, the tendency of the armature flux to decarapidly is resisted by an increasing flux threading the short circuitedturns, which flux is caused by virtue of the voltage induced therein.This induced voltage in the short circuited turns is, in the examplgiven, the

product of multiplying the resistance of the short-circuited turns (10.9ohms) by the armature current (4 amperes) by the transformation ratio(12). The voltage induced is 523 volts. The summation of current in theshort-circuited turns at time of separation of the brush is 48 amperes,It creates a flux which opposes the decay of armature flux in order tominimize the inductive voltage of the armature. In other words, theshort-circuited turns on the armature operate like the short-circuitedturns of a slowactlng or time-lag relay. Anthough Fig. 3 indicates only16 main coils M, it will be understood that, when 32 main coils areused, there will be 32 commutator bars and each core slot will containfour active coil sides of different main coils. The commutator barsconnected with short-circuited turns are spaced by bars not soconnected.

Connection of the short circuited turns to the commutator is formechanical reasons only. The commutator bars or risers act merely as aconvenient support for the joints of the short-circuited coils and serveto resist the action of centrifugal force.

While the embodiment of the present invention as herein disclosed,constitutes a preferred form, it is to be understood that other formsmight be adopted, all coming within the scope of the claim whichfollows.

What is claimed is as follows:

A direct current dynamo electric machine having an armature comprising acore having teeth spaced by slots, a commutator, brushes engageable withthe commutator, multi-turn coils connected with the commutator andsurrounding a group of -core teeth, and short-circuited damping coilsinterlinking all of the multi-turn coils, each damping coil spanning thegroup of core teeth surrounded by a multi-turn coil, said damping coilsserving to reduce the inductive voltage which is caused by theinterruption of the load current in the inductive circuit of thearmature due to brush vibration.

RAYMOND H. SULLIVAN.

